RISC microprocessor architecture implementing multiple typed register sets

ABSTRACT

A register system for a data processor which operates in a plurality of modes. The register system provides multiple, identical banks of register sets, the data processor controlling access such that instructions and processes need not specify any given bank. An integer register set includes first (RA[23:0]) and second (RA[31:24]) subsets, and a shadow subset (RT[31:24]). While the data processor is in a first mode, instructions access the first and second subsets. While the data processor is in a second mode, instructions may access the first subset, but any attempts to access the second subset are re-routed to the shadow subset instead, transparently to the instructions, allowing system routines to seemingly use the second subset without having to save and restore data which user routines have written to the second subset. A re-typable register set provides integer width data and floating point width data in response to integer instructions and floating point instructions, respectively. Boolean comparison instructions specify particular integer or floating point registers for source data to be compared, and specify a particular Boolean register for the result, so there are no dedicated, fixed-location status flags. Boolean combinational instructions combine specified Boolean registers, for performing complex Boolean comparisons without intervening conditional branch instructions, to minimize pipeline disruption.

CROSS-REFERENCE TO RELATED APPLICATIONS

Applications of particular interest to the present application, include:

1. HIGH-PERFORMANCE RISC MICROPROCESSOR ARCHITECTURE, SC/Ser. No.07/817,810, filed Jan. 8, 1992, which is a continuation of Ser. No.07/727,066, filed Jul. 8, 1991, by Le Nguyen, et al.;

2. EXTENSIBLE RISC MICROPROCESSOR ARCHITECTURE, SC/Ser. No. 07/817,809,filed Jan. 8, 1992, which is a continuation of Ser. No. 07/727,058,filed Jul. 8, 1991, by Quang Trang, et al.

3. RISC MICROPROCESSOR ARCHITECTURE WITH ISOLATED ARCHITECTURALDEPENDENCIES, SC/Ser. No. 07/817,807, filed Jan. 8, 1992, which is acontinuation of Ser. No. 07/726,744, filed Jul. 8, 1991, by YoshiMiyayama;

4. RISC MICROPROCESSOR ARCHITECTURE IMPLEMENTING FAST TRAP AND EXCEPTIONSTATE, Ser. No. 08/345,333 which is now U.S. Pat. No. 5,481,685, filedNov. 21, 1994, which is a continuation of Ser. No. 07/171,968, filedDec. 23, 1993, which is a continuation of Ser. No. 07/817,811, filedJan. 8, 1992, which is a continuation of Ser. No. 07/726,942, filed Jul.8, 1991 by Quang Trang;

5. SINGLE CHIP PAGE PRINTER CONTROLLER, SC/Ser. No. 07/817,813, filedJan. 8, 1992, which is a continuation of Ser. No. 07/726,929, filed Jul.8, 1991 by Derek Lentz; and

6. MICROPROCESSOR ARCHITECTURE CAPABLE OF SUPPORTING MULTIPLEHETEROGENEOUS PROCESSORS, SC/Ser. No. 07/726,893, filed Jul. 8, 1991, byDerek Lentz.

The above-identified Applications are hereby incorporated herein byreference, their collective teachings being part of the presentdisclosure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to microprocessors, and morespecifically to a RISC microprocessor having plural, symmetrical sets ofregisters.

2. Description of the Background

In addition to the usual complement of main memory storage and secondarypermanent storage, a microprocessor-based computer system typically alsoincludes one or more general purpose data registers, one or more addressregisters, and one or more status flags. Previous systems have includedinteger registers for holding integer data and floating point registersfor holding floating point data. Typically, the status flags are usedfor indicating certain conditions resulting from the most recentlyexecuted operation. There generally are status flags for indicatingwhether, in the previous operation: a carry occurred, a negative numberresulted, and/or a zero resulted.

These flags prove useful in determining the outcome of conditionalbranching within the flow of program control. For example, if it isdesired to compare a first number to a second number and upon theconditions that the two are equal, to branch to a given subroutine, themicroprocessor may compare the two numbers by subtracting one from theother, and setting or clearing the appropriate condition flags. Thenumerical value of the result of the subtraction need not be stored. Aconditional branch instruction may then be executed, conditioned uponthe status of the zero flag. While being simple to implement, thisscheme lacks flexibility and power. Once the comparison has beenperformed, no further numerical or other operations may be performedbefore the conditional branch upon the appropriate flag; otherwise, theintervening instructions will overwrite the condition flag valuesresulting from the comparison, likely causing erroneous branching. Thescheme is further complicated by the fact that it may be desirable toform greatly complex tests for branching, rather than the simpleequality example given above.

For example, assume that the program should branch to the subroutineonly upon the condition that a first number is greater than a secondnumber, and a third number is less than a fourth number, and a fifthnumber is equal to a sixth number. It would be necessary for previousmicroprocessors to perform a lengthy series of comparisons heavilyinterspersed with conditional branches. A particularly undesirablefeature of this serial scheme of comparing and branching is observed inany microprocessor having an instruction pipeline.

In a pipelined microprocessor, more than one instruction is beingexecuted at any given time, with the plural instructions being indifferent stages of execution at any given moment. This provides forvastly improved throughput. A typical pipeline microprocessor mayinclude pipeline stages for: (a) fetching an instruction, (b) decodingthe instruction, (c) obtaining the instruction's operands, (d) executingthe instruction, and (e) storing the results. The problem arises when aconditional branch instruction is fetched. It may be the case that theconditional branch's condition cannot yet be tested, as the operands maynot yet be calculated, if they are to result from operations which areyet in the pipeline. This results in a "pipeline stall" whichdramatically slows down the processor.

Another shortcoming of previous microprocessor-based systems is thatthey have included only a single set of registers of any given datatype. In previous architectures, when an increased number of registershas been desired within a given data type, the solution has been simplyto increase the size of the single set of those type of registers. Thismay result in addressing problems, access conflict problems, andsymmetry problems.

On a similar note, previous architectures have restricted each givenregister set to one respective numerical data type. Various priorsystems have allowed general purpose registers to hold either numericaldata or address "data", but the present application will not use theterm "data" to include addresses. What is intended may be bestunderstood with reference to two prior systems. The Intel 8085microprocessor includes a register pair "HL" which can be used to holdeither two bytes of numerical data or one two-byte address. The presentapplication's improvement is not directed to that issue. More on point,the Intel 80486 microprocessor includes a set of general purpose integerdata registers and a set of floating point registers, with each setbeing limited to its respective data type, at least for purposes ofdirect register usage by arithmetic and logic units.

This proves wasteful of the microprocessor's resources, such as theavailable silicon area, when the microprocessor is performing operationswhich do not involve both data types. For example, user applicationsfrequently involve exclusively integer operations, and perform nofloating point operations whatsoever. When such a user application isrun on a previous microprocessor which includes floating point registers(such as the 80486), those floating point registers remain idle duringthe entire execution.

Another problem with previous microprocessor register set architectureis observed in context switching or state switching between a userapplication and a higher access privilege level entity such as theoperating system kernel. When control within the microprocessor switchescontext, mode, or state, the operating system kernel or other entity towhich control is passed typically does not operate on the same datawhich the user application has been operating on. Thus, the dataregisters typically hold data values which are not useful to the newcontrol entity but which must be maintained until the user applicationis resumed. The kernel must generally have registers for its own use,but typically has no way of knowing which registers are presently in useby the user application. In order to make space for its own data, thekernel must swap out or otherwise store the contents of a predeterminedsubset of the registers. This results in considerable loss of processingtime to overhead, especially if the kernel makes repeated,short-duration assertions of control.

On a related note, in prior microprocessors, when it is required that a"grand scale" context switch be made, it has been necessary for themicroprocessor to expend even greater amounts of processing resources,including a generally large number of processing cycles, to save alldata and state information before making the switch. When context isswitched back, the same performance penalty has previously been paid, torestore the system to its former state. For example, if a microprocessoris executing two user applications, each of which requires the fullcomplement of registers of each data type, and each of which may be invarious stages of condition code setting operations or numericalcalculations, each switch from one user application to the othernecessarily involves swapping or otherwise saving the contents of everydata register and state flag in the system. This obviously involves agreat deal of operational overhead, resulting in significant performancedegradation, particularly if the main or the secondary storage to whichthe registers must be saved is significantly slower than themicroprocessor itself.

Therefore, we have discovered that it is desirable to have an improvedmicroprocessor architecture which allows the various componentconditions of a complex condition to be calculated without anyintervening conditional branches. We have further discovered that it isdesirable that the plural simple conditions be calculable in parallel,to improve throughput of the microprocessor.

We have also discovered that it is desirable to have an architecturewhich allows multiple register sets within a given data type.

Additionally, we have discovered it to be desirable for amicroprocessor's floating point registers to be usable as integerregisters, in case the available integer registers are inadequate tooptimally to hold the necessary amount of integer data. Notably, we havediscovered that it is desirable that such re-typing be completelytransparent to the user application.

We have discovered it to be highly desirable to have a microprocessorwhich provides a dedicated subset of registers which are reserved foruse by the kernel in lieu of at least a subset of the user registers,and that this new set of registers should be addressable in exactly thesame manner as the register subset which they replace, in order that thekernel may use the same register addressing scheme as user applications.We have further observed that it is desirable that the switch betweenthe two subsets of registers require no microprocessor overhead cycles,in order to maximally utilize the microprocessor's resources.

Also, we have discovered it to be desirable to have a microprocessorarchitecture which allows for a "grand scale" context switch to beperformed with minimal overhead. In this vein, we have discovered thatis desirable to have an architecture which allows for plural banks ofregister sets of each type, such that two or more user applications maybe operating in a multi-tasking environment, or other "simultaneous"mode, with each user application having sole access to at least a fullbank of registers. It is our discovery that the register addressingscheme should, desirably, not differ between user applications, norbetween register banks, to maximize simplicity of the user applications,and that the system should provide hardware support for switchingbetween the register banks so that the user applications need not beaware of which register bank which they are presently using or even ofthe existence of other register banks or of other user applications.

These and other advantages of our invention will be appreciated withreference to the following description of our invention, theaccompanying drawings, and the claims.

SUMMARY OF THE INVENTION

The present invention provides a register file system comprising: aninteger register set including first and second subsets of integerregisters, and a shadow subset; a re-typable set of registers which areindividually usable as integer registers or as floating point registers;and a set of individually addressable Boolean registers.

The present invention includes integer and floating point functionalunits which execute integer instructions accessing the integer registerset, and which operate in a plurality of modes. In any mode,instructions are granted ordinary access to the first subset of integerregisters. In a first mode, instructions are also granted ordinaryaccess to the second subset. However, in a second mode, instructionsattempting to access the second subset are instead granted access to theshadow subset, in a manner which is transparent to the instructions.Thus, routines may be written without regard to which mode they willoperate in, and system routines (which operate in the second mode) canhave at least the second subset seemingly at their disposal, withouthaving to expend the otherwise-required overhead of saving the secondsubset's contents (which may be in use by user processes operating inthe first mode).

The invention further includes a plurality of integer register sets,which are individually addressable as specified by fields ininstructions. The register sets include read ports and write ports whichare accessed by multiplexers, wherein the multiplexers are controlled bycontents of the register set-specifying fields in the instructions.

One of the integer register sets is also usable as a floating pointregister set. In one embodiment, this set is sixty-four bits wide tohold double-precision floating point data, but only the low orderthirty-two bits are used by integer instructions.

The invention includes functional units for performing Booleanoperations, and further includes a Boolean register set for holdingresults of the Boolean operations such that no dedicated, fixed-locationstatus flags are required. The integer and floating point functionalunits execute numerical comparison instructions, which specifyindividual ones of the Boolean registers to hold results of thecomparisons. A Boolean functional unit executes Boolean combinationalinstructions whose sources and destination are specified registers inthe Boolean register set. Thus, the present invention may performconditional branches upon a single result of a complex Boolean functionwithout intervening conditional branch instructions between thefundamental parts of the complex Boolean function, minimizing pipelinedisruption in the data processor.

Finally, there are multiple, identical register banks in the system,each bank including the above-described register sets. A bank may beallocated to a given process or routine, such that the instructionswithin the routine need not specify upon which bank they operate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the instruction execution unit of themicroprocessor of the present invention, showing the elements of theregister file.

FIGS. 2, 3 and 4 are simplified schematic and block diagrams of thefloating point, integer and Boolean portions of the instructionexecution unit of FIG. 1, respectively.

FIGS. 5-6 are more detailed views of the floating point and integerportions, respectively, showing the means for selecting between registersets.

FIG. 7 illustrates the fields of an exemplary microprocessor instructionword executable by the instruction execution unit of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS I. REGISTER FILE

FIG. 1 illustrates the basic components of the instruction executionunit (IEU) 10 of the RISC (reduced instruction set computing) processorof the present invention. The IEU 10 includes a register file 12 and anexecution engine 14. The register file 12 includes one or more registerbanks 16-0 to 16-n. It will be understood that the structure of eachregister bank 16 is identical to all of the other register banks 16.Therefore, the present application will describe only register bank16-0. The register bank includes a register set A 18, a register set FB20, and a register set C 22.

In general, the invention may be characterized as a RISC microprocessorhaving a register file optimally configured for use in the execution ofRISC instructions, as opposed to conventional register files which aresufficient for use in the execution of CISC (complex instruction setcomputing) instructions by CISC processors. By having a speciallyadapted register file, the execution engine of the microprocessor's IEUachieves greatly improved performance, both in terms of resourceutilization and in terms of raw throughput. The general concept is totune a register set to a RISC instruction, while the specificimplementation may involve any of the register sets in the architecture.

A. Register Set A

Register set A 18 includes integer registers 24 (RA[31:0]), each ofwhich is adapted to hold an integer value datum. In one embodiment, eachinteger may be thirty-two bits wide. The RA[] integer registers 24include a first plurality 26 of integer registers (RA[23:0]) and asecond plurality 28 of integer registers (RA[31:24]). The RA[] integerregisters 24 are each of identical structure, and are each addressablein the same manner, albeit with a unique address within the integerregister set 24. For example, a first integer register 30 (RA[0]) isaddressable at a zero offset within the integer register set 24.

RA[0] always contains the value zero. It has been observed that userapplications and other programs use the constant value zero more thanany other constant value. It is, therefore, desirable to have a zeroreadily available at all times, for clearing, comparing, and otherpurposes. Another advantage of having a constant, hard-wired value in agiven register, regardless of the particular value, is that the givenregister may be used as the destination of any instruction whose resultsneed not be saved.

Also, this means that the fixed register will never be the cause of adata dependency delay. A data dependency exists when a "slave"instruction requires, for one or more of its operands, the result of a"master" instruction. In a pipelined processor, this may cause pipelinestalls. For example, the master instruction, although occurring earlierin the code sequence than the slave instruction, may take considerablylonger to execute. It will be readily appreciated that if a slave"increment and store" instruction operates on the result data of amaster "quadruple-word integer divide" instruction, the slaveinstruction will be fetched, decoded, and awaiting execution many clockcycles before the master instruction has finished execution. However, incertain instances, the numerical result of a master instruction is notneeded, and the master instruction is executed for some other purposeonly, such as to set condition code flags. If the master instruction'sdestination is RA[0], the numerical results will be effectivelydiscarded. The data dependency checker (not shown) of the IEU 10 willnot cause the slave instruction to be delayed, as the ultimate result ofthe master instruction --zero-- is already known.

The integer register set A 24 also includes a set of shadow registers 32(RT[31:24]). Each shadow register can hold an integer value, and is, inone embodiment, also thirty-two bits wide. Each shadow register isaddressable as an offset in the same manner in which each integerregister is addressable.

Finally, the register set A includes an IEU mode integer switch 34. Theswitch 34, like other such elements, need not have a physical embodimentas a switch, so long as the corresponding logical functionality isprovided within the register sets. The IEU mode integer switch 34 iscoupled to the first subset 26 of integer registers on line 36, to thesecond subset of integer registers 28 on line 38, and to the shadowregisters 32 on line 40. All accesses to the register set A 18 are madethrough the IEU mode integer switch 34 on line 42. Any access request toread or write a register in the first subset RA[23:0] is passedautomatically through the IEU mode integer switch 34. However, accessesto an integer register with an offset outside the first subset RA[23:0]will be directed either to the second subset RA[31:24] or the shadowregisters RT[31;24], depending upon the operational mode of theexecution engine 14.

The IEU mode integer switch 34 is responsive to a mode control unit 44in the execution engine 14. The mode control unit 44 provides pertinentstate or mode information about the IEU 10 to the IEU mode integerswitch 34 on line 46. When the execution engine performs a contextswitch such as a transfer to kernel mode, the mode control unit 44controls the IEU mode integer switch 34 such that any requests to thesecond subset RA[31:24] are re-directed to the shadow RT[31:24], usingthe same requested offset within the integer set. Any operating systemkernel or other then-executing entity may thus have apparent access tothe second subset RA[31:24] without the otherwise-required overhead ofswapping the contents of the second subset RA[31:24] out to main memory,or pushing the second subset RA[31:24] onto a stack, or otherconventional register-saving technique.

When the execution engine 14 returns to normal user mode and controlpasses to the originally-executing user application, the mode controlunit 44 controls the IEU mode integer switch 34 such that access isagain directed to the second subset RA[31:24]. In one embodiment, themode control unit 44 is responsive to the present state of interruptenablement in the IEU 10. In one embodiment, the execution engine 14includes a processor status register (PSR) (not shown), which includes aone-bit flag (PSR[7]) indicating whether interrupts are enabled ordisabled. Thus, the line 46 may simply couple the IEU mode integerswitch 34 to the interrupts-enabled flag in the PSR. While interruptsare disabled, the IEU 10 maintains access to the integers RA[23:0], inorder that it may readily perform analysis of various data of the userapplication. This may allow improved debugging, error reporting, orsystem performance analysis.

B. Register Set FB

The re-typable register set FB 20 may be thought of as includingfloating point registers 48 (RF[31:0]), and/or integer registers 50(RB[31:0]). When neither data type is implied to the exclusion of theother, this application will use the term RFB[]. In one embodiment, thefloating point registers RF[] occupy the same physical silicon space asthe integer registers RB[]. In one embodiment, the floating pointregisters RF[] are sixty-four bits wide and the integer registers RB[]are thirty-two bits wide. It will be understood that if double-precisionfloating point numbers are not required, the register set RFB[] mayadvantageously be constructed in a thirty-two-bit width to save thesilicon area otherwise required by the extra thirty-two bits of eachfloating point register.

Each individual register in the register set RFB[] may hold either afloating point value or an integer value. The register set RFB[] mayinclude optional hardware for preventing accidental access of a floatingpoint value as though it were an integer value, and vice versa. In oneembodiment, however, in the interest of simplifying the register setRFB[], it is simply left to the software designer to ensure that noerroneous usages of individual registers are made. Thus, the executionengine 14 simply makes an access request on line 52, specifying anoffset into the register set RFB[], without specifying whether theregister at the given offset is intended to be used as a floating pointregister or an integer register. Within the execution engine 14, variousentities may use either the full sixty-four bits provided by theregister set RFB[], or may use only the low order thirty-two bits, suchas in integer operations or single-precision floating point operations.

A first register RFB[0] 51 contains the constant value zero, in a formsuch that RB[0] is a thirty-two-bit integer zero (0000_(hex)) and RF[0]is a sixty-four-bit floating point zero (00000000_(hex)). This providesthe same advantages as described above for RA[0].

C. Register Set C

The register set C 22 includes a plurality of Boolean registers 54(RC[31:0]). RC[] is also known as the "condition status register" (CSR).The Boolean registers RC[] are each identical in structure andaddressing, albeit that each is individually addressable at a uniqueaddress or offset within RC[].

In one embodiment, register set C further includes a "previous conditionstatus register" (PCSR) 60, and the register set C also includes a CSRselector unit 62, which is responsive to the mode control unit 44 toselect alternatively between the CSR 54 and the PCSR 60. In the oneembodiment, the CSR is used when interrupts are enabled, and the PCSR isused when interrupts are disabled. The CSR and PCSR are identical in allother respects. In the one embodiment, when interrupts are set to bedisabled, the CSR selector unit 62 pushes the contents of the CSR intothe PCSR, overwriting the former contents of the PCSR, and wheninterrupts are re-enabled, the CSR selector unit 62 pops the contents ofthe PCSR back into the CSR. In other embodiments it may be desirable tomerely alternate access between the CSR and the PCSR, as is done withRA[31:24] and RT[31:24]. In any event, the PCSR is always available as athirty-two-bit "special register".

None of the Boolean registers is a dedicated condition flag, unlike theBoolean registers in previously known microprocessors. That is, the CSR54 does not include a dedicated carry flag, nor a dedicated a minusflag, nor a dedicated flag indicating equality of a comparison or a zerosubtraction result. Rather, any Boolean register may be the destinationof the Boolean result of any Boolean operation. As with the otherregister sets, a first Boolean register 58 (RC[0]) always contains thevalue zero, to obtain the advantages explained above for RA[0]. In thepreferred embodiment, each Boolean register is one bit wide, indicatingone Boolean value.

II. EXECUTION ENGINE

The execution engine 14 includes one or more integer functional units66, one or more floating point functional units 68, and one or moreBoolean functional units 70. The functional units execute instructionsas will be explained below. Buses 72, 73, and 75 connect the variouselements of the IEU 10, and will each be understood to represent data,address, and control paths.

A. Instruction Format

FIG. 7 illustrates one exemplary format for an integer instruction whichthe execution engine 14 may execute. It will be understood that not allinstructions need to adhere strictly to the illustrated format, and thatthe data processing system includes an instruction fetcher and decoder(not shown) which are adapted to operate upon varying formatinstructions. The single example of FIG. 7 is for ease in explanationonly. Throughout this Application the identification I[] will be used toidentify various bits of the instruction. I[31:30] are reserved forfuture implementations of the execution engine 14. I[29:26] identify theinstruction class of the particular instruction. Table 1 shows thevarious classes of instructions performed by the present invention.

                  TABLE 1                                                         ______________________________________                                        Instruction Classes                                                           Class         Instructions                                                    ______________________________________                                        0-3           Integer and floating point                                                    register-to-register instructions                                4            Immediate constant load                                          5            Reserved                                                         6            Load                                                             7            Store                                                            8-11         Control Flow                                                    12            Modifier                                                        13            Boolean operations                                              14            Reserved                                                        15            Atomic (extended)                                               ______________________________________                                    

Instruction classes of particular interest to this Application includethe Class 0-3 register-to-register instructions and the Class 13 Booleanoperations. While other classes of instructions also operate upon theregister file 12, further discussion of those classes is not believednecessary in order to fully understand the present invention.

I[25] is identified as B0, and indicates whether the destinationregister is in register set A or register set B. I[24:22] are an opcodewhich identifies, within the given instruction class, which specificfunction is to be performed. For example, within theregister-to-register classes, an opcode may specify "addition". I[21]identifies the addressing mode which is to be used when performing theinstruction--either register source addressing or immediate sourceaddressing. I[20:16] identify the destination register as an offsetwithin the register set indicated by B0. ]2115] is identified as B1 andindicates whether the first operand is to be taken from register set Aor register set B. I[14:10] identify the register offset from which thefirst operand is to be taken. I[9:8] identify a function selection--anextension of the opcode I[24:22]. I[7:6] are reserved. I[5] isidentified as B2 and indicates whether a second operand of theinstruction is to be taken from register set A or register set B.Finally, I[4:0] identify the register offset from which the secondoperand is to be taken.

With reference to FIG. 1, the integer functional unit 66 and floatingpoint functional unit 68 are equipped to perform integer comparisoninstructions and floating point comparisons, respectively. Theinstruction format for the comparison instruction is substantiallyidentical to that shown in FIG. 7, with the caveat that various fieldsmay advantageously be identified by slightly different names. I[20:16]identifies the destination register where the result is to be stored,but the addressing mode field I[21] does not select between registersets A or B. Rather, the addressing mode field indicates whether thesecond source of the comparison is found in a register or is immediatedata. Because the comparison is a Boolean type instruction, thedestination register is always found in register set C. All other fieldsfunction as shown in FIG. 7. In performing Boolean operations within theinteger and floating point functional units, the opcode and functionselect fields identify which Boolean condition is to be tested for incomparing the two operands. The integer and the floating pointfunctional units fully support the IEEE standards for numericalcomparisons.

The IEU 10 is a load/store machine. This means that when the contents ofa register are stored to memory or read from memory, an addresscalculation must be performed in order to determine which location inmemory is to be the source or the destination of the store or load,respectively. When this is the case, the destination register fieldI[20:16] identifies the register which is the destination or the sourceof the load or store, respectively. The source register 1 field,I[14:10], identifies a register in either set A or B which contains abase address of the memory location. In one embodiment, the sourceregister 2 field, I[4:0], identifies a register in set A or set B whichcontains an index or an offset from the base. The load/store address iscalculated by adding the index to the base. In another mode, I[7:0]include immediate data which are to be added as an index to the base.

B. Operation of the Instruction Execution Unit and Register Sets

It will be understood by those skilled in the art that the integerfunctional unit 66, the floating point functional unit 68, and theBoolean functional unit 70 are responsive to the contents of theinstruction class field, the opcode field, and the function select fieldof a present instruction being executed.

1. Integer Operations

For example, when the instruction class, the opcode, and function selectindicate that an integer register-to-register addition is to beperformed, the integer functional unit may be responsive thereto toperform the indicated operation, while the floating point functionalunit and the Boolean functional unit may be responsive thereto to notperform the operation. As will be understood from the cross-referencedapplications, however, the floating point functional unit 68 is equippedto perform both floating point and integer operations. Also, thefunctional units are constructed to each perform more than oneinstruction simultaneously.

The integer functional unit 66 performs integer functions only. Integeroperations typically involve a first source, a second source, and adestination. A given integer instruction will specify a particularoperation to be performed on one or more source operands and willspecify that the result of the integer operation is to be stored at agiven destination. In some instructions, such as address calculationsemployed in load/store operations, the sources are utilized as a baseand an index. The integer functional unit 66 is coupled to a first bus72 over which the integer functional unit 66 is connected to a switchingand multiplexing control (SMC) unit A 74 and an SMC unit B 76. Eachinteger instruction executed by the integer functional unit 66 willspecify whether each of its sources and destination reside in registerset A or register set B.

Suppose that the IEU 10 has received, from the instruction fetch unit(not shown), an instruction to perform an integer register-to-registeraddition. In various embodiments, the instruction may specify a registerbank, perhaps even a separate bank for each source and destination. Inone embodiment, the instruction I[] is limited to a thirty-two-bitlength, and does not contain any indication of which register bank 16-0through 16-n is involved in the instruction. Rather, the bank selectorunit 78 controls which register bank is presently active. In oneembodiment, the bank selector unit 78 is responsive to one or more bankselection bits in a status word (not shown) within the IEU 10.

In order to perform the integer addition instruction, the integerfunctional unit 66 is responsive to the identification in I[14:10] andI[4:0] of the first and second source registers. The integer functionalunit 66 places an identification of the first and second sourceregisters at ports S1 and S2, respectively, onto the integer functionalunit bus 72 which is coupled to both SMC units A and B 74 and 76. In oneembodiment, the SMC units A and B are each coupled to receive B0-2 fromthe instruction I[]. In one embodiment, a zero in any respective Bnindicates register set A, and a one indicates register set B. Duringload/store operations, the source ports of the integer and floatingpoint functional units 66 and 68 are utilized as a base port and anindex port, B and I, respectively.

After obtaining the first and second operands from the indicatedregister sets on the bus 72, as explained below, the integer functionalunit 66 performs the indicated operation upon those operands, andprovides the result at port D onto the integer functional unit bus 72.The SMC units A and B are responsive to B0 to route the result to theappropriate register set A or B.

The SMC unit B is further responsive to the instruction class, opcode,and function selection to control whether operands are read from (orresults are stored to) either a floating point register RF[] or aninteger register RB[]. As indicated, in one embodiment, the registersRF[] may be sixty-four bits wide while the registers are RB[] are onlythirty-two bits wide. Thus, SMC unit B controls whether a word or adouble word is written to the register set RFB[]. Because all registerswithin register set A are thirty-two bits wide, SMC unit, A need notinclude means for controlling the width of data transfer on the bus 42.

All data on the bus 42 are thirty-two bits wide, but other sorts ofcomplexities exist within register set A. The IEU mode integer switch 34is responsive to the mode control unit 44 of the execution engine 14 tocontrol whether data on the bus 42 are connected through to bus 36, bus38 or bus 40, and vice versa.

IEU mode integer switch 34 is further responsive to I[20:16], I[14:10],and I[4:0]. If a given indicated destination or source is in RA[23:0],the IEU mode integer switch 34 automatically couples the data betweenlines 42 and 36. However, for registers RA[31:24], the IEU mode integerswitch 34 determines whether data on line 42 is connected to line 38 orline 40, and vice versa. When interrupts are enabled, IEU mode integerswitch 34 connects the SMC unit A to the second subset 28 of integerregisters RA[31:24]. When interrupts are disabled, the IEU mode integerswitch 34 connects the SMC unit A to the shadow registers RT[31:24].Thus, an instruction executing within the integer functional unit 66need not be concerned with whether to address RA[31:24] or RT[31:24]. Itwill be understood that SMC unit A may advantageously operateidentically whether it is being accessed by the integer functional unit66 or by the floating point functional unit 68.

2. Floating Point Operations

The floating point functional unit 68 is responsive to the class,opcode, and function select fields of the instruction, to performfloating point operations. The S1, S2, and D ports operate as describedfor the integer functional unit 66. SMC unit B is responsive to retrievefloating point operands from, and to write numerical floating pointresults to, the floating point registers RF[] on bus 52.

3. Boolean Operations

SMC unit C 80 is responsive to the instruction class, opcode, andfunction select fields of the instruction I[]. When SMC unit C detectsthat a comparison operation has been performed by one of the numericalfunctional units 66 or 68, it writes the Boolean result over bus 56 tothe Boolean register indicated at the D port of the functional unitwhich performed the comparison.

The Boolean functional unit 70 does not perform comparison instructionsas do the integer and floating point functional units 66 and 68. Rather,the Boolean functional unit 70 is only used in performing bitwiselogical combination of Boolean register contents, according to theBoolean functions listed in Table 2.

                  TABLE 2                                                         ______________________________________                                        Boolean Functions                                                             I[23,22,9,8]    Boolean result calculation                                    ______________________________________                                        0000            ZERO                                                          0001            S1 AND S2                                                     0010            S1 AND (NOT S2)                                               0011            S1                                                            0100            (NOT S1) AND S2                                               0101            S2                                                            0110            S1 XOR S2                                                     0111            S1 OR S2                                                      1000            S1 NOR S2                                                     1001            S1 XNOR S2                                                    1010            NOT S2                                                        1011            S1 OR (NOT S2)                                                1100            NOT S1                                                        1101            (NOT S1) OR S2                                                1110            S1 NAND S2                                                    1111            ONE                                                           ______________________________________                                    

The advantage which the present invention obtains by having a pluralityof homogenous Boolean registers, each of which is individuallyaddressable as the destination of a Boolean operation, will be explainedwith reference to Tables 3-5. Table 3 illustrates an example of asegment of code which performs a conditional branch based upon a complexBoolean function. The complex Boolean function includes three portionswhich are OR-ed together. The first portion includes two sub-portions,which are AND-ed together.

                  TABLE 3                                                         ______________________________________                                        Example of Complex Boolean Function                                           1      RA[1] := 0;                                                            2      IF (((RA[2] = RA[3]) AND (RA[4] > RA[5])) OR                           3        (RA[6] < RA[7]) OR                                                   4        (RA[8] <> RA[9])) THEN                                               5        X( )                                                                 6      ELSE                                                                   7        Y( );                                                                8      RA[10] := 1;                                                           ______________________________________                                    

Table 4 illustrates, in pseudo-assembly form, one likely method by whichprevious microprocessors would perform the function of Table 3. The codein Table 4 is written as though it were constructed by a compiler of atleast normal intelligence operating upon the code of Table 3. That is,the compiler will recognize that the condition expressed in lines 2-4 ofTable 3 is passed if any of the three portions is true.

                  TABLE 4                                                         ______________________________________                                        Execution of Complex Boolean Function                                         Without Boolean Register Set                                                  1      START        LDI      RA[1],0                                          2      TEST1        CMP      RA[2],RA[3]                                      3                   BNE      TEST2                                            4                   CMP      RA[4],RA[5]                                      5                   BGT      DO.sub.-- IF                                     6      TEST2        CMP      RA[6],RA[7]                                      7                   BLT      DO.sub.-- IF                                     8      TEST3        CMP      RA[8],RA[9]                                      9                   BEQ      DO.sub.-- ELSE                                   10     DO.sub.-- IF JSR      ADDRESS OF X( )                                  11                  JMP      PAST.sub.-- ELSE                                 12     DO.sub.-- ELSE                                                                             JSR      ADDRESS OF Y( )                                  13     PAST.sub.-- ELSE                                                                           LDI      RA[10],1                                         ______________________________________                                    

The assignment at line 1 of Table 3 is performed by the "load immediate"statement at line 1 of Table 4. The first portion of the complex Booleancondition, expressed at line 2 of Table 3, is represented by thestatements in lines 2-5 of Table 4. To test whether RA[2] equals RA[3],the compare statement at line 2 of Table 4 performs a subtraction ofRA[2]from RA[3] or vice versa, depending upon the implementation, andmay or may not store the result of that subtraction. The importantfunction performed by the comparison statement is that the zero, minus,and carry flags will be appropriately set or cleared.

The conditional branch statement at line 3 of Table 4 branches to asubsequent portion of code upon the condition that RA[2] did not equalRA[3]. If the two were unequal, the zero flag will be clear, and thereis no need to perform the second sub-portion. The existence of theconditional branch statement at line 3 of Table 4 prevents the furtherfetching, decoding, and executing of any subsequent statement in Table 4until the results of the comparison in line 2 are known, causing apipeline stall. If the first sub-portion of the first portion (TEST1) ispassed, the second sub-portion at line 4 of Table 4 then compares RA[4]to RA[5], again setting and clearing the appropriate status flags.

If RA[2] equals RA[3], and RA[4] is greater than RA[5], there is no needto test the remaining two portions (TEST2 and TEST3) in the complexBoolean function, and the statement at Table 4, line 5, willconditionally branch to the label DO₋₋ IF, to perform the operationinside the "IF" of Table 3. However, if the first portion of the test isfailed, additional processing is required to determine which of the "IF"and "ELSE" portions should be executed.

The second portion of the Boolean function is the comparison of RA[6] toRA[7], at line 6 of Table 4, which again sets and clears the appropriatestatus flags. If the condition "less than" is indicated by the statusflags, the complex Boolean function is passed, and execution mayimmediately branch to the DO₋₋ IF label. In various priormicroprocessors, the "less than" condition may be tested by examiningthe minus flag. If RA[7]was not less than RA[6], the third portion ofthe test must be performed. The statement at line 8 of Table 4 comparesRA[8] to RA[9]. If this comparison is failed, the "ELSE" code should beexecuted; otherwise, execution may simply fall through to the "IF" codeat line 10 of Table 4, which is followed by an additional jump aroundthe "ELSE" code. Each of the conditional branches in Table 4, at lines3, 5, 7 and 9, results in a separate pipeline stall, significantlyincreasing the processing time required for handling this complexBoolean function.

The greatly improved throughput which results from employing the Booleanregister set C of the present invention will now readily be seen withspecific reference to Table 5.

                  TABLE 5                                                         ______________________________________                                        Execution of Complex Boolean Function                                         With Boolean Register Set                                                     1     START        LDI      RA[1],0                                           2     TEST1        CMP      RC[11],RA[2],RA[3],EQ                             3                  CMP      RC[12],RA[4],RA[5],GT                             4     TEST2        CMP      RC[13],RA[6],RA[7],LT                             5     TEST3        CMP      RC[14],RA[8],RA[9],NE                             6     COMPLEX      AND      RC[15],RC[11],RC[12]                              7                  OR       RC[16],RC[13],RC[14]                              8                  OR       RC[17],RC[15],RC[16]                              9                  BC       RC[17],DO.sub.-- ELSE                             10    DO.sub.-- IF JSR      ADDRESS OF X( )                                   11                 JMP      PAST.sub.-- ELSE                                  12    DO.sub.-- ELSE                                                                             JSR      ADDRESS OF Y( )                                   13    PAST.sub.-- ELSE                                                                           LDI      RA[10],1                                          ______________________________________                                    

Most notably seen at lines 2-5 of Table 5, the Boolean register set Callows the microprocessor to perform the three test portionsback-to-back without intervening branching. Each Boolean comparisonspecifies two operands, a destination, and a Boolean condition for whichto test. For example, the comparison at line 2 of Table 5 compares thecontents of RA[2] to the contents of RA[3], tests them for equality, andstores into RC[11] the Boolean value of the result of the comparison.Note that each comparison of the Boolean function stores its respectiveintermediate results in a separate Boolean register. As will beunderstood with reference to the above-referenced related applications,the IEU 10 is capable of simultaneously performing more than one of thecomparisons.

After at least the first two comparisons at lines 2-3 of Table 5 havebeen completed, the two respective comparison results are AND-edtogether as shown at line 6 of Table 3. RC[15] then holds the result ofthe first portion of the test. The results of the second and thirdsub-portions of the Boolean function are OR-ed together as seen in Table5, line 7. It will be understood that, because there are no datadependencies involved, the AND at line 6 and the OR-ed in line 7 may beperformed in parallel. Finally, the results of those two operations areOR-ed together as seen at line 8 of Table 5. It will be understood thatregister RC[17] will then contain a Boolean value indicating the truthor falsity of the entire complex Boolean function of Table 3. It is thenpossible to perform a single conditional branch, shown at line 9 ofTable 5. In the mode shown in Table 5, the method branches to the "ELSE"code if Boolean register RC[17] is clear, indicating that the complexfunction was failed. The remainder of the code may be the same as it waswithout the Boolean register set as seen in Table 4.

The Boolean functional unit 70 is responsive to the instruction class,opcode, and function select fields as are the other functional units.Thus, it will be understood with reference to Table 5 again, that theinteger and/or floating point functional units will perform theinstructions in lines 1-5 and 13, and the Boolean functional unit 70will perform the Boolean bitwise combination instructions in lines 6-8.The control flow and branching instructions in line 9-12 will beperformed by elements of the IEU 10 which are not shown in FIG. 1.

III. DATA PATHS

FIGS. 2-5 illustrate further details of the data paths within thefloating point, integer, and Boolean portions of the IEU, respectively.

A. Floating Point Portion Data Paths

As seen in FIG. 2, the register set FB 20 is a multi-ported registerset. In one embodiment, the register set FB 20 has two write portsWFB0-1, and five read ports RDFB0-4. The floating point functional unit68 of FIG. 1 is comprised of the ALU2 102, FALU 104, MULT 106, and NULL108 of FIG. 2. All elements of FIG. 2 except the register set 20 and theelements 102-108 comprise the SMC unit B of FIG. 1.

External, bidirectional data bus EX₋₋ DATA[] provides data to thefloating point load/store unit 122. Immediate floating point data busLDF₋₋ IMED[] provides data from a "load immediate" instruction. Otherimmediate floating point data are provided on busses RFF1₋₋ IMED andRFF2₋₋ IMED, such as is involved in an "add immediate" instruction. Dataare also provided on bus EX₋₋ SR₋₋ DT[], in response to a "specialregister move" instruction. Data may also arrive from the integerportion, shown in FIG. 3, on busses 114 and 120.

The floating point register set's two write ports WFB0 and WFB1 arecoupled to write multiplexers 110-0 and 110-1, respectively. The writemultiplexers 110 receive data from: the ALU0 or SHF0 of the integerportion of FIG. 3; the FALU; the MULT; the ALU2; either EX₋₋ SR₋₋ DT[]or LDF₋₋ IMED[]; and EX₋₋ DATA[]. Those skilled in the art willunderstand that control signals (not shown) determine which input isselected at each port, and address signals (not shown) determine towhich register the input data are written. Multiplexer control andregister addressing are within the skill of persons in the art, and willnot be discussed for any multiplexer or register set in the presentinvention.

The floating point register set's five read ports RDFB0 to RDFB4 arecoupled to read multiplexers 112-0 to 112-4, respectively. The readmultiplexers each also receives data from: either EX₋₋ SR₋₋ DT[] orLDF₋₋ IMED[], on load immediate bypass bus 126; a load external databypass bus 127, which allows external load data to skip the register setFB; the output of the ALU2 102, which performs non-multiplicationinteger operations; the FALU 104, which performs non-multiplicationfloating point operations; the MULT 106, which performs multiplicationoperations; and either the ALU0 140 or the SHF0 144 of the integerportion shown in FIG. 3, which respectively perform non-multiplicationinteger operations and shift operations. Read multiplexers 112-1 and112-3 also receive data from RFF1₋₋ IMED[] and RFF2₋₋ IMED[],respectively.

Each arithmetic-type unit 102-106 in the floating point portion receivestwo inputs, from respective sets of first and second source multiplexersS1 and S2. The first source of each unit ALU2, FALU, and MULT comes fromthe output of either read multiplexer 112-0 or 112-2, and the secondsource comes from the output of either read multiplexer 112-1 or 112-3.The sources of the FALU and the MULT may also come from the integerportion of FIG. 3 on bus 114.

The results of the ALU2, FALU, and MULT are provided back to the writemultiplexers 110 for storage into the floating point registers RF[], andalso to the read multiplexers 112 for re-use as operands of subsequentoperations. The FALU also outputs a signal FALU₋₋ BD indicating theBoolean result of a floating point comparison operation. FALU₋₋ BD iscalculated directly from internal zero and sign flags of the FALU.

Null byte tester NULL 108 performs null byte testing operations upon anoperand from a first source multiplexer, in one mode that of the ALU2.NULL 108 outputs a Boolean signal NULLB₋₋ BD indicating whether thethirty-two-bit first source operand includes a byte of value zero.

The outputs of read multiplexers 112-0, 112-1, and 112-4 are provided tothe integer portion (of FIG. 3) on bus 118. The output of readmultiplexer 112-4 is also provided as STDT₋₋ FP[]store data to thefloating point load/store unit 122.

FIG. 5 illustrates further details of the control of the S1 and S2multiplexers. As seen, in one embodiment, each S1 multiplexer may beresponsive to bit B1 of the instruction I[], and each S2 multiplexer maybe responsive to bit B2 of the instruction I[]. The S1 and S2multiplexers select the sources for the various functional units. Thesources may come from either of the register files, as controlled by theB1 and B2 bits of the instruction itself. Additionally, each registerfile includes two read ports from which the sources may come, ascontrolled by hardware not shown in the Figs.

B. Integer Portion Data Paths

As seen in FIG. 3, the register set A 18 is also multi-ported. In oneembodiment, the register set A 18 has two write ports WA0-1, and fiveread ports RDA0-4. The integer functional unit 66 of FIG. 1 is comprisedof the ALU0 140, ALU1 142, SHF0 144, and NULL 146 of FIG. 3. Allelements of FIG. 3 except the register set 18 and the elements 140-146comprise the SMC unit A of FIG. 1.

External data bus EX₋₋ ₋₋ DATA[] provides data to the integer load/storeunit 152. Immediate integer data on bus LDI₋₋ IMED[] are provided inresponse to a "load immediate" instruction. Other immediate integer dataare provided on busses RFA1₋₋ IMED and RFA2₋₋ IMED in response tonon-load immediate instructions, such as an "add immediate". Data arealso provided on bus EX₋₋ SR₋₋ DT[] in response to a "special registermove" instruction. Data may also arrive from the floating point portion(shown in FIG. 2) on busses 116 and 118.

The integer register set's two write ports WA0 and WA1 are coupled towrite multiplexers 148-0 and 148-1, respectively. The write multiplexers148 receive data from: the FALU or MULT of the floating point portion(of FIG. 2); the ALU0; the ALU1; the SHF0; either EX₋₋ SR₋₋ DT[] orLDI₋₋ IMED[]; and EX₋₋ DATA[].

The integer register set's five read ports RDA0 to RDA4 are coupled toread multiplexers 150-0 to 150-4, respectively. Each read multiplexeralso receives data from: either EX₋₋ SR₋₋ DT[] or LDI₋₋ IMED[] on loadimmediate bypass bus 160; a load external data bypass bus 154, whichallows external load data to skip the register set A; ALU0; ALU1; SHF0;and either the FALU or the MULT of the floating point portion (of FIG.2). Read multiplexers 150-1 and 150-3 also receive data from RFA1₋₋IMED[]and RFA2₋₋ IMED[], respectively.

Each arithmetic-type unit 140-144 in the integer portion receives twoinputs, from respective sets of first and second source multiplexers S1and S2. The first source of ALU0 comes from either the output of readmultiplexer 150-2, or a thirty-two-bit wide constant zero (0000_(hex)),or floating point read multiplexer 112-4. The second source of ALU0comes from either read multiplexer 150-3 or floating point readmultiplexer 112-1. The first source of ALU1 comes from either readmultiplexer 150-0 or IF₋₋ PC[]. IF₋₋ PC[] is used in calculating areturn address needed by the instruction fetch unit (not shown), due tothe IEU's ability to perform instructions in an out-of-order sequence.The second source of ALU1 comes from either read multiplexer 150-1 orCF₋₋ OFFSET[]. CF₋₋ OFFSET[] is used in calculating a return address fora CALL instruction, also due to the out-of-order capability.

The first source of the shifter SHF0 144 is from either: floating pointread multiplexer 112-0 or 112-4; or any integer read multiplexer 150.The second source of SHF0 is from either: floating point readmultiplexer 112-0 or 112-4; or integer read multiplexer 150-0, 150-2, or150-4. SHF0 takes a third input from a shift amount multiplexer (SA).The third input controls how far to shift, and is taken by the SAmultiplexer from either: floating point read multiplexer 112-1; integerread multiplexer 150-1 or 150-3; or a five-bit wide constant thirty-one(11111₂ or 31₁₀). The shifter SHF0 requires a fourth input from the sizemultiplexer (S). The fourth input controls how much data to shift, andis taken by the S multiplexer from either: read multiplexer 150-1; readmultiplexer 150-3; or a five-bit wide constant sixteen (10000₂ or 16₁₀).

The results of the ALU0, ALU1, and SHF0 are provided back to the writemultiplexers 148 for storage into the integer registers RA[], and alsoto the read multiplexers 150 for re-use as operands of subsequentoperations. The output of either ALU0 or SHF0 is provided on bus 120 tothe floating point portion of FIG. 3. The ALU0 and ALU1 also outputsignals ALU0₋₋ BD and ALU1₋₋ BD, respectively, indicating the Booleanresults of integer comparison operations. ALU0₋₋ BD and ALU1₋₋ BD arecalculated directly from the zero and sign flags of the respectivefunctional units. ALU0 also outputs signals EX₋₋ TADR[] and EX₋₋ VM₋₋ADR. EX₋₋ TADR[] is the target address generated for an absolute branchinstruction, and is sent to the IFU (not shown) for fetching the targetinstruction. EX₋₋ VM₋₋ ADR[] is the virtual address used for all loadsfrom memory and stores to memory, and is sent to the VMU (not shown) foraddress translation.

Null byte tester NULL 146 performs null byte testing operations upon anoperand from a first source multiplexer. In one embodiment, the operandis from the ALU0. NULL 146 outputs a Boolean signal NULLA₋₋ BDindicating whether the thirty-two-bit first source operand includes abyte of value zero.

The outputs of read multiplexers 150-0 and 150-1 are provided to thefloating point portion (of FIG. 2) on bus 114. The output of readmultiplexer 150-4 is also provided as STDT₋₋ INT[] store data to theinteger load/store unit 152.

A control bit PSR[7] is provided to the register set A 18. It is thissignal which, in FIG. 1, is provided from the mode control unit 44 tothe IEU mode integer switch 34 on line 46. The IEU mode integer switchis internal to the register set A 18 as shown in FIG. 3.

FIG. 6 illustrates further details of the control of the S1 and S2multiplexers. The signal ALU0₋₋ BD

C. Boolean Portion Data Paths

As seen in FIG. 4, the register set C 22 is also multi-ported. In oneembodiment, the register set C 22 has two write ports WC0-1, and fiveread ports RDA0-4. All elements of FIG. 4 except the register set 22 andthe Boolean combinational unit 70 comprise the SMC unit C of FIG. 1.

The Boolean register set's two write ports WC0 and WC1 are coupled towrite multiplexers 170-0 and 170-1, respectively. The write multiplexers170 receive data from: the output of the Boolean combinational unit 70,indicating the Boolean result of a Boolean combinational operation;ALU0₋₋ BD from the integer portion of FIG. 3, indicating the Booleanresult of an integer comparison; FALU₋₋ BD from the floating pointportion of FIG. 2, indicating the Boolean result of a floating pointcomparison; either ALU1₋₋ BD₋₋ P from ALU1, indicating the results of acompare instruction in ALU1, or NULLA₋₋ BD from NULL 146, indicating anull byte in the integer portion; and either ALU2₋₋ BD₋₋ P from ALU2,indicating the results of a compare operation in ALU2, or NULLB₋₋ BDfrom NULL 108, indicating a null byte in the floating point portion. Inone mode, the ALU0₋₋ BD, ALU1₋₋ BD, ALU2₋₋ BD, and FALU₋₋ BD signals arenot taken from the data paths, but are calculated as a function of thezero flag, minus flag, carry flag, and other condition flags in the PSR.In one mode, wherein up to eight instructions may be executing at oneinstant in the IEU, the IEU maintains up to eight PSRs.

The Boolean register set C is also coupled to bus EX₋₋ SR₋₋ DT[], foruse with "special register move" instructions. The CSR may be written orread as a whole, as though it were a single thirty-two-bit register.This enables rapid saving and restoration of machine state information,such as may be necessary upon certain drastic system errors or uponcertain forms of grand scale context switching.

The Boolean register set's five read ports RDC0 to RDC3 are coupled toread multiplexers 172-0 to 172-4, respectively. The read multiplexers172 receive the same set of inputs as the write multiplexers 170receive. The Boolean combinational unit 70 receives inputs from readmultiplexers 170-0 and 170-1. Read multiplexers 172-2 and 172-3respectively provide signals BLBP₋₋ CPORT and BLBP₋₋ DPORT. BLBP₋₋ CPORTis used as the basis for conditional branching instructions in the IEU.BLBP₋₋ DPORT is used in the "add with Boolean" instruction, which setsan integer register in the A or B set to zero or one (with leadingzeroes), depending upon the content of a register in the C set. Readport RDC4 is presently unused, and is reserved for future enhancementsof the Boolean functionality of the IEU.

IV. CONCLUSION

While the features and advantages of the present invention have beendescribed with respect to particular embodiments thereof, and in varyingdegrees of detail, it will be appreciated that the invention is notlimited to the described embodiments. The following claims define theinvention to be afforded patent coverage.

We claim:
 1. In a data processing system, which includes a centralprocessing unit (CPU) which performs operations according to aninstruction, the operations operating upon integer data, a data registersystem comprising:a first register set including a plurality of firstregisters each for holding the integer data; a second register setincluding a plurality of second registers each for holding the integerdata and for holding floating point data, wherein the instructionincludes a field specifying which of the first and second register setsis to be accessed in response to the instruction; means, responsive tothe field, for accessing the first register set or the second registerset as specified by the field, includingi) reading means for reading anoperand value from either the first register set or second register setas specified by the field, and ii) writing mean for writing a resultvalue to the first register set or the second register set as specifiedby the field.
 2. The apparatus of claim 1, wherein said first and secondregister sets each have two write ports and five read ports.
 3. Anapparatus comprising:execution means for executing instructions, theinstructions performing operations upon operands to generate results,each instruction specifying a respective source address for each operandand a destination address for the result of the instruction, eachaddress specifying a register set and an offset; a first register setincluding a plurality of individually addressable registers each forstoring integer values; first access means for writing and readingvalues to and from the first register set according to a giveninstruction, the first access means including,i) first reading means,responsive to the given instruction having a given source address whichspecifies the first register set as a source for an operand of the giveninstruction, for reading the operand's value from the first register setat the offset specified by the given source address, and ii) firstwriting means, responsive to the given instruction having a givendestination address which specifies the first register set as adestination for the result of the given instruction, for writing theresult's value to the first register set at the offset specified by thegiven destination address; a second register set including a pluralityof individually addressable registers, wherein each addressable registeris configured to store floating point values and integer values; andsecond access means for writing and reading integer and floating pointvalues to and from the second register set according to the giveninstruction, the second access means including,i) second reading means,responsive to the given instruction having a given source address whichspecifies the second register set as a source for an operand of thegiven instruction, for reading the operand's value from the secondregister set at the offset specified by the given source address, andii) second writing means, responsive to the given instruction having agiven destination address which specifies the second register set as adestination for the result of the given instruction, for writing theresult's value to the second register set at the offset specified by thegiven destination address.
 4. The apparatus of claim 3, wherein:a giveninstruction may specify a first and a second source address and adestination address, with each address specifying either of the first orsecond register sets such that the given instruction requires access toboth register sets; and the first and second access means operatesimultaneously to provide the instruction parallel access to both thefirst and second register sets.
 5. The apparatus of claim 2, whereinsaid first and second register sets each have two write ports and fiveread ports.